Massive Multiple-Input Multiple-Output (massive-MIMO) radio transmission is a technique for wireless access, e.g., in telecommunications networks like or beyond Long Term Evolution (LTE). Hundreds of phase-coherently operating base station antennas serve many tens of user equipments on the same time-frequency resource.
Each user equipment needs to transmit its own pilot signal (also referred to as reference signal) in a coherence interval. The pilot signal allows the radio base station (e.g., an eNodeB in an LTE network) to measure Channel State Information (CSI) for the coherence interval. The massive-MIMO transmission uses the CSI for closed-loop beamforming, so that the number of user equipments that can be served per radio base station is significantly increased. However, the increased user density leads to an increase in the rate of user equipments accessing the radio base station for the first time, e.g., because the user equipment has been turned on or the user equipment has entered the coverage area of the radio base station. Furthermore, the closed-loop beamforming in massive-MIMO transmissions allows for high-mobility channels (e.g., for user equipments located in a high-speed train), which further adds to the rate of first-time accesses at the radio base station. For the first-time access, the user equipments select their pilot signals, e.g., randomly. The same pilot signal may be selected by different user equipments in the same coherence interval for the first-time access, since the number of different pilot signals is limited while the number of accessing user equipments is increasing. Hence, access bursts with colliding pilot signals can be expected to become more frequent.
Conventionally, user equipments transmitting the same pilot signal have to reattempt until their access bursts are collision-free, which causes delay and occupies radio resources. Increasing the length of the pilot signals would increase the number of distinct pilot signals. However, the longer the pilot signals the more radio capacity is occupied and unavailable for payload data.